The present invention relates to an objective lens which is used for an optical disc drive capable of recording/reading data to/from two or more types of optical discs having different data densities and cover layer thicknesses.
There exist many types of optical discs according to various standards with different data densities and different thicknesses of the cover layer (protective layer). For instance, DVD (Digital Versatile Disc) has higher record density and a thinner cover layer than CD (Compact Disc). Therefore, when an optical disc loaded on an optical disc drive is replaced with another optical disc of a different type (standard), another effective beam diameter suitable for data density of the new disc has to be attained for reading/writing information while properly correcting spherical aberration which changes depending on the cover layer thickness.
For example, for realizing the information recording/readout on optical discs having high data density, a beam spot diameter has to be reduced by increasing an NA (numerical aperture) in comparison with an optical system specially employed for optical discs having low data density. Since the beam spot diameter gets smaller as the beam wavelength gets shorter, a laser light source emitting a laser beam having a wavelength of 635-665 nm has to be employed for optical systems for DVD, which is shorter than that (780–830 nm) of optical systems exclusively used for CD. Therefore, optical disc drives (optical information recording/readout devices) of recent years are provided with an optical system having a light source unit capable of emitting laser beams of different wavelengths.
In order for properly converging and focusing the laser beam on recording surfaces of optical discs having different cover layer thicknesses, an objective lens having a diffracting structure (which is partitioned into a plurality of annular zones by small level differences) on its one side is in practical use today for the optical systems for optical discs. Such an objective lens focuses the laser beam on the recording surface of the disc constantly with a proper NA even when optical discs of different standards are used, utilizing the characteristics of the diffracting structure causing different spherical aberrations depending on the wavelength of the incident beam.
The surface of such a conventional objective lens is divided into an inner area including the optical axis of the lens and an outer area outside the inner area. Typically, the inner area has diffracting structure capable of properly focusing the beam for the CD on the recording surface of the CD while also being capable of properly focusing the beam for the DVD on the recording surface of the DVD. Meanwhile, the outer area has diffracting structure capable of properly focusing the beam for the DVD on the recording surface of the DVD while being capable of preventing the proper focusing of the beam for the CD on the recording surface of the CD and thereby avoiding excessive convergence of the beam.
That is, part of the beam for the CD that passed through the outer area is diffused on the recording surface due to large spherical aberration caused by the diffracting structure, while only part of the beam for the CD that passed through the inner area is focused on the recording surface and forms a beam spot of a relatively large diameter. In the case of the beam for DVD, the NA becomes larger since part of the beam passing through the outer area also focuses on the recording surface, by which a small beam spot suitable for information recording/readout of DVD having high data density is formed on the recording surface.
Such a DVD/CD compatible objective lens and an optical information recording/readout device employing the DVD/CD compatible objective lens have been disclosed in Japanese Patent Provisional Publications No. P2001-216674A and P2001-249273A, for example.
In the case of writable optical discs such as CD-R and CD-RW, the diameter of the beam spot (on the recording surface) for writing is desired to be smaller than that for reading. The spot diameter can be reduced by using a beam of a short wavelength, setting the NA large, etc. However, the CD-R requires a long-wavelength beam of 780 nm or more due to reflection characteristics of its recording surface, therefore, the NA is set relatively large in order to realize the optimum spot size for recording the information onto the CD-R (RW). The large NA can be attained by, for example, increasing the effective diameter of the objective lens when optical discs of the CD standard are used, that is, by enlarging the inner area of the objective lens.
However, in the conventional DVD/CD compatible objective lens, the enlargement of the inner area for optimizing the information recording/readout of the CD-R (RW) might cause deterioration of wavelength characteristics or temperature characteristics when the beam for recording/readout of DVD is incident on the objective lens. Here, the “wavelength characteristics” of the objective lens means the extent of the change of spherical aberration when the wavelength of the incident beam changes. With deteriorated wavelength characteristics, even a slight change in the incident beam wavelength (due to individual differences of the light source, etc.) causes large spherical aberration. The “temperature characteristics” of the objective lens means the extent of the change of spherical aberration that is caused by the change of temperature. With deteriorated temperature characteristics, even a slight change in temperature (due to environmental variation, etc.) results in large spherical aberration.
In general, the permissible range of spherical aberration in reading/writing information becomes smaller as the data density of the optical disc increases. Therefore, the conventional DVD/CD compatible objective lens, designed to have a large inner area, has only a little margin for the permissible range, and thus further improvement of the wavelength characteristics and temperature characteristics has been desired.